Reducing Power Consumption and Resource Utilization During Miss Lookahead

1. A method for executing program instructions on a processor, comprising: issuing instructions for execution during a normal-execution mode; upon encountering a triggering instruction which causes an unresolved data dependency, speculatively executing subsequent instructions in a lookahead mode to prefetch future load; and when the processor determines that the lookahead mode is unlikely to uncover an additional outer-level cache miss, terminating the lookahead mode and recommencing execution at the triggering instruction in the normal-execution mode after the unresolved data dependency is resolved, wherein the processor determines that the lookahead mode is unlikely to uncover an additional outer-level cache miss based on at least one of: determining that the lookahead mode uncovers the same number of cache misses as were uncovered during a previous lookahead mode which originated from the same triggering instruction; determining that the lookahead mode executes the same number of instructions as were executed until a last outer-level cache miss was uncovered during the previous lookahead mode; and determining that the lookahead mode encounters N branches, wherein the N branches were unresolvable because of data dependency problems and were predicted by a branch predictor with low confidence.

2. The method of claim 1 , wherein the processor uses a program counter for the triggering instruction to perform a table lookup to determine the number of cache misses which were uncovered during the previous lookahead mode which originated from the same triggering instruction.

3. The method of claim 1 , wherein if no outer-level cache misses were uncovered during the previous lookahead mode which originated from the same triggering instruction, the lookahead mode is not entered.

4. The method of claim 1 , wherein the processor uses a program counter for the triggering instruction to perform a table lookup to determine the number of instructions which were executed until a last outer-level cache miss was uncovered during a previous lookahead mode which originated from the same triggering instruction.

5. The method of claim 1 , wherein if no outer-level cache misses were uncovered during the previous lookahead mode which originated from the same triggering instruction, the lookahead mode is not entered.

6. The method of claim 1 , wherein prior to speculatively executing the instructions in the lookahead mode, the method further comprises generating a checkpoint that can subsequently be used to return execution of the program to the point of the triggering instruction.

7. The method of claim 1 , wherein when the unresolved data dependency that originally caused the system to move out of normal-execution mode is finally resolved, the method further comprises resuming execution in the normal-execution mode from the triggering instruction.

8. The method of claim 1 , wherein the unresolved data dependency can include: a use of an operand that has not returned from a preceding load miss; a use of an operand that has not returned from a preceding translation lookaside buffer (TLB) miss; a use of an operand that has not returned from a preceding full or partial read-after-write (RAW) from store buffer operation; and a use of an operand that depends on another operand that is subject to an unresolved data dependency.

9. An apparatus that executes program instructions, comprising: an execution mechanism configured to issue instructions for execution during a normal-execution mode; and a detection mechanism configured to detect an unresolved data dependency; wherein if an unresolved data dependency is detected during execution of a triggering instruction by a strand, the execution mechanism is configured to speculatively execute subsequent instructions in a lookahead mode to prefetch future loads; and wherein if the execution mechanism determines that the lookahead mode is unlikely to uncover an additional outer-level cache miss, the execution mechanism is configured to terminate the lookahead mode and recommence execution at the triggering instruction in the normal-execution mode after the unresolved data dependency is resolved, wherein the execution mechanism determines that the lookahead mode is unlikely to uncover an additional outer-level cache miss based on at least one of: determining that the lookahead mode uncovers the same number of cache misses as were uncovered during a previous lookahead mode which originated from the same triggering instruction; determining that the lookahead mode executes the same number of instructions as were executed until a last outer-level cache miss was uncovered during the previous lookahead mode; and determining that the lookahead mode encounters N branches, wherein the N branches were unresolvable because of data dependency problems and were predicted by a branch predictor with low confidence.

10. The apparatus of claim 9 , wherein the execution mechanism is configured to use a program counter for the triggering instruction to perform a table lookup to determine the number of cache misses which were uncovered during the previous lookahead mode which originated from the same triggering instruction.

11. The apparatus of claim 9 , wherein if no outer-level cache misses were uncovered during the previous lookahead mode which originated from the same triggering instruction, the strand does not enter the lookahead mode.

12. The apparatus of claim 9 , wherein the execution mechanism is configured to use a program counter for the triggering instruction to perform a table lookup to determine the number of instructions which were executed until a last outer-level cache miss was uncovered during a previous lookahead mode which originated from the same triggering instruction.

13. The apparatus of claim 9 , wherein if no outer-level cache misses were uncovered during the previous lookahead mode which originated from the same triggering instruction, the strand does not enter the lookahead mode.

14. The apparatus of claim 9 , wherein prior to speculatively executing the instructions in the lookahead mode, the execution mechanism is configured to generate a checkpoint that can subsequently be used to return execution of the program to the point of the triggering instruction.

15. The apparatus of claim 9 , wherein when the unresolved data dependency that originally caused the system to move out of normal-execution mode is finally resolved, the execution mechanism is configured to resume execution in the normal-execution mode from the triggering instruction.

16. A method for executing program instructions on a processor, comprising: issuing instructions for execution during a normal-execution mode; upon encountering a triggering instruction which causes an unresolved data dependency, speculatively executing subsequent instructions in a lookahead mode to prefetch future loads; and wherein the processor treats strands in the lookahead mode with lower priority than strands in the normal-execution mode by at least of: allocating fewer resources to strands in the lookahead mode as compared to strands in the normal-operating mode; and dropping prefetch requests for strands in the lookahead mode before dropping prefetch requests for strands in the normal-operating mode.

17. The method of claim 16 , wherein allocating fewer resources to strands in the lookahead mode as compared to strands in the normal-operating mode involves limiting a strand in the lookahead mode to using fewer pick queue entries than a strand in the normal-operating mode.

18. An apparatus that executes program instructions, comprising: an execution mechanism configured to issue instructions for execution during a normal-execution mode; and a detection mechanism configured to detect an unresolved data dependency; wherein if an unresolved data dependency is detected during execution of a triggering instruction by a strand, the execution mechanism is configured to speculatively execute subsequent instructions in a lookahead mode to prefetch future loads; and wherein the execution mechanism is configured to treat strands in the lookahead mode with lower priority than strands in the normal-execution mode by at least one of: allocating fewer resources to strands in the lookahead mode as compared to strands in the normal-operating mode; dropping prefetch requests for strands in the lookahead mode before dropping prefetch requests for strands in the normal-operating mode.

19. The apparatus of claim 18 , wherein allocating fewer resources to strands in the lookahead mode as compared to strands in the normal-operating mode involves limiting a strand in the lookahead mode to using fewer pick queue entries than a strand in the normal-operating mode.